This study will investigate the changes in cognitive and behavioral development as a consequence of the birth of a sibling. A conceptual framework integrating sociological, developmental, psychological and economic models of family dynamics is formulated. The birth of a sibling may influence the developmental trajectory of a child partially because it affects the availability of developmental resources of the family to the older children. A multivariate analytical model is proposed, that identifies the effects of birth of a sibling on several developmental outcomes, controlling for the initial level of development and the compositional differences between the children who are likely to experience the birth of a sibling versus those who are not. Furthermore, this study will identify the subgroups of children who are most vulnerable to the negative effects of birth of a sibling, the age intervals during which the birth of a sibling is more likely to have negative effects on development, and the developmental domains (i.e. behavioral or cognitive) that are most vulnerable to the negative effects of birth of a sibling. Longitudinal data from the 1986, 1988 and 1990 rounds of the Children of the National Longitudinal Survey of Youth (NLSY) will be used. These data provide detailed longitudinal information on children's cognitive and behavioral development, family histories of the children, maternal characteristics, and resources provided specifically to each child. The focal children of this study are children born to a national sample of a cohort of women who, on average have completed the first half of their childbearing years by 1990. Two cohorts of children will be considered: Children who were 0 to 2 years old, and children who were 3 to 5 years old in 1986. Births of siblings during 1986-88 and 1988-90 intervals will be considered.